System and technique for characterizing fluids using ultrasonic diffraction grating spectroscopy

ABSTRACT

A system for determining property of multiphase fluids based on ultrasonic diffraction grating spectroscopy includes a diffraction grating on a solid in contact with the fluid. An interrogation device delivers ultrasound through the solid and a captures a reflection spectrum from the diffraction grating. The reflection spectrum exhibits peaks whose relative size depends on the properties of the various phases of the multiphase fluid. For example, for particles in a liquid, the peaks exhibit dependence on the particle size and the particle volume fraction. Where the exact relationship is know know a priori, data from different peaks of the same reflection spectrum or data from the peaks of different spectra obtained from different diffraction gratings can be used to resolve the size and volume fraction.

RELATED APPLICATION DATA

The present application is a continuation-in-part of U.S. applicationSer. No. 10/430,474 filed May 6, 2003, now U.S. Pat. No. 6,877,375,which claims the benefit of commonly owned U.S. Provisional ApplicationSer. No. 60/378,530 filed May 6, 2002 and commonly owned U.S.Provisional Application Ser. No 60/467,878 filed May 5, 2003 and titledCharacterization of Fluids and Slurries Using Ultrasonic DiffractionGrating Spectroscopy. The present application claims the benefit of U.S.Provisional Application Ser. No. 60/644,758 filed Jan. 17, 2005. Thepresent application is related to commonly owned U.S. application Ser.No. 10/099,412 filed Mar. 15, 2002 and titled Self Calibrating Systemand Technique for Ultrasonic Determination of Fluid Properties. Thedisclosures of the above referenced applications are all herebyincorporated by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract NumberDE-AC0576RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND

The present invention relates to fluid analysis and more particularly,but not exclusively, relates to the determination of fluid properties bydetecting ultrasound reflected from a diffraction grating.

Fluids are encountered in a wide variety of industrial applications, andthere is a continual need to determine properties of those fluids.Ultrasound based sensors have been developed for a variety of industrialapplications, but there continues to be a need to develop improvedsensors and sensor techniques for determining fluid properties. Inparticular there is a need for sensing systems and techniques that areaccurate, reliable, cost effective and can be implemented in a widevariety of industrial applications. The present invention is addressedto these needs and provides novel systems and techniques for determiningfluid properties utilizing ultrasonic diffraction grating spectroscopy.In particular embodiments, the present invention may be used incharacterizing multiphase fluids, for example solid liquid mixtures,such as slurries, suspensions and the like.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic view of a system for determining fluidproperties via multiple reflections from a fluid-solid interface.

FIG. 2 is a schematic view of a device for performing an ultrasonictime-of-flight measurement on a fluid.

FIG. 3 is a schematic view of another device for performing anultrasonic time-of-flight measurement on a fluid.

FIG. 4 is a diagrammatic view of a variation of the FIG. 1 system fordetermining fluid properties.

FIG. 5 is a side view of a clamp on sensor attached to a pipeline.

FIG. 6 is a sectional view of the FIG. 5 sensor.

FIG. 7 is an exemplary plot of echo magnitude versus time illustratingechoes 1-5 of a representative diminishing series of echo amplitudes.

FIG. 8 is an exemplary plot of log echo amplitude versus echo numberwith a straight line fit to the exemplary data where m indicates theslope of the line.

FIG. 9 is a perspective view of an acoustic system for determining fluidproperties implemented as a spool piece that can be coupled to a processline. The acoustic system of FIG. 9 is a combination sensor thatincludes a pair of transducers for performing sensing based on multiplereflections from a fluid-solid interface and a pair of transducers forperforming ultrasonic diffraction grating spectroscopy.

FIG. 10 is a schematic illustration of a system for detecting thereflection spectrum including the zero order diffraction.

FIG. 11 is an exemplary schematic illustration of the relative locationof reflected and transmitted zero order waves for a stainless steelwater interface with a diffraction period of 300 μm and illustrating thefrequency dependence of the relative orientation of the diffracted firstorder waves.

FIG. 12 is a schematic illustration of an ultrasonic beam incident on adiffraction grating.

FIG. 13 is a schematic illustration of a system for performingultrasonic diffraction grating spectroscopy.

FIG. 14 is a plot of reflection coefficient as a function of frequencyfor the zero order diffraction collected with an exemplary embodiment asdescribed where the fluid is water.

FIG. 15 is a plot of reflection coefficient as a function of frequencyfor the zero order diffraction collected with an exemplary embodiment asdescribed where the fluid is 10% sugar water.

FIG. 16 is a plot of reflection coefficient as a function of frequencyfor the zero order diffraction collected with an exemplary embodiment asdescribed where the fluid is 15% sugar water.

FIGS. 17 and 18 are plots of normalized transducer response versusfrequency for various concentrations of water and 215 micron polystyrenespheres.

FIGS. 19 and 20 are plots of normalized transducer response versusfrequency for various concentrations of water and 275 micron polystyrenespheres.

FIG. 21 are plots of normalized transducer response versus frequency forvarious concentrations of water and 363 micron polystyrene spheres.

FIG. 22 are plots of normalized transducer response versus frequency forvarious concentrations of water and 463 micron polystyrene spheres.

FIG. 23 is a plot of normalized transducer response versus frequency for11 wt % mixtures of polystyrene spheres at different average particlesizes.

FIG. 24 is a plot of peak height minus the background versus weightpercentage for the various polystyrene spheres.

FIG. 25 is a plot of the slope of the straight line curve fits of FIG.24 versus particle size.

FIG. 26 are plots of area under the peak versus weight percentage forthe slurries of all four particle diameters.

FIG. 27 is a plot of the slopes of the straight line curve fits of FIG.26 versus the particle diameter.

FIG. 28 are plots of the percentage of ultrasound absorbed versus weightpercent for the different particle sizes.

FIG. 29 is a plot of the percentage of ultrasound absorbed versusparticle diameter.

FIGS. 30 a and 30 b are calibration curves showing the interdependenceof peak height above background (PKHB) on the ordinate, the weightpercentage, and the particle size for the aluminum grating (FIG. 30 a)and for a second different grating (FIG. 30 b).

FIG. 31 is a plot of the substitution of the values into Equation 19 asdescribed in the specification.

DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended. Any alterations and further modificationsin the described embodiments, and any further applications of theprinciples of the invention as described herein are contemplated aswould normally occur to one skilled in the art to which the inventionrelates.

In one aspect, the present invention provides a sensor for determining aproperty of a fluid utilizing ultrasonic diffraction gratingspectroscopy. Briefly, this technique involves a diffraction gratingformed on a solid that is placed in contact with a fluid. In oneapplication, ultrasound is transmitted through the solid at an angle ofincidence and a reflection of the ultrasound from the diffractiongrating is collected. The reflected ultrasound exhibits a frequencydependent response that can be correlated with properties of the fluidin contact with the diffraction grating. For example, a criticalfrequency has been found that manifests as an identifiable peak in aplot of amplitude versus frequency for the reflected ultrasoundcorresponding to zero order diffraction. The location of this peak, i.e.the frequency value, has been found to correlate with acoustic velocity(i.e. speed of sound) in the fluid in both a qualitative andquantitative manner.

In a further aspect, the sensor of the present invention can be used tocharacterize multiphase systems, for example a suspension of solidparticles in a liquid.

Turning first to FIGS. 10-13, system 400 includes a member 410 comprisedof solid material having a diffraction grating 450 formed on a facethereof and positioned in contact with a fluid 420. A send transducer430 is acoustically coupled to the solid member 410 and is configured todirect ultrasound through the member 410 and onto the diffractiongrating 450 at an angle of incidence θ measured relative to a normal 460of the grating 450. A receive transducer is acoustically coupled to themember 410 and receives ultrasound which reflects from the grating 450.As explained more fully below, the reflection spectrum from the grating450 exhibits spatial and frequency dependence that can be correlated toproperties of the fluid 420. In the schematic illustration of FIG. 10,the receive transducer 440 is positioned to receive the zero orderreflection (designated m=0) which occurs at an angle of reflection θequal to the angle of incidence θ regardless of frequency. In othervariations, the receiver transducer 440 is positioned opposite the sendtransducer 430 relative to the normal 460 but at an angle either greaterthan or less than the angle of incidence θ. In still other variations,the angle θ is zero and a single transducer functions as both the sendand receive transducers.

FIG. 12 shows an ultrasonic beam of frequency f traveling through asolid, and striking a grating-liquid interface at an incident angle θ.As a result of constructive interference, a refracted beam in the liquidis produced at angle φ_(m). The grating period is d and isconventionally defined as the distance between adjacent grooves in theultrasonic diffraction grating. (For simplicity, the grooves are shownschematically in FIG. 12 as “slits” though it is understood that agrating can take any periodic form, for example grooves of uniformshape, such as triangular, square, or serpentine) In the solid, thespeed of a longitudinal sound wave is c_(L) and the wavelength is λ₁. Inthe liquid the corresponding speed of sound is c and the wavelength isλ. Constructive interference occurs when:

$\begin{matrix}{{{\frac{OC}{\lambda} - \frac{AB}{\lambda_{1}}} = m}{{\frac{{df}\;\sin\mspace{11mu}\varphi_{m}}{c} - \frac{{df}\;\sin\mspace{11mu}\theta}{c_{L}}} = m}} & (9)\end{matrix}$where m is zero, or a positive or negative integer. When m=0, Snell'sLaw is obtained:

$\begin{matrix}{\frac{\;{\sin\mspace{11mu}\varphi_{0}}}{c} = \frac{\;{\sin\mspace{11mu}\theta}}{c_{L}}} & (10)\end{matrix}$Using the results of Eq. (10), Eq. (9) becomes

$\begin{matrix}{{{\sin\mspace{11mu}\varphi_{m}} - \;{\sin\mspace{11mu}\varphi_{0}}} = \frac{mc}{fd}} & (11)\end{matrix}$Eq. (11) is the so-called grating equation. When m=+1, Eq. (11)determines the angle φ_(m). Table 1 shows the frequency required toplace the transmitted longitudinal wave at angle φ_(m) for an exemplarystainless steel-water interface and a grating period (d) of 300 μm.

Order m Frequency (MHz) Angle φ_(m) (degrees) 0 Any value 7.35 1 6.35 651 6.10 70 1 5.90 70 1 5.90 75 1 5.75 80 1 5.70 84 1 5.67 90Note that as the frequency decreases, the angle φ₁ increases. When thefrequency is 5.67 MHz, the angle φ₁ reaches 90°, which is termed acritical frequency F_(CR). Setting φ₁=90° and rearranging Eq. (11)yields an expression for this critical frequency (F_(CR)):

$\begin{matrix}{F_{CR} = \frac{c}{d( {1 - {\sin\;\varphi_{0}}} )}} & (12)\end{matrix}$

Solving Eq. (10) for sin φ₀ and substituting into Eq. (12) yields anexpression for the critical frequency F_(CR) as a function of d, c,c_(L) and θ:F _(CR) =c/{d[1−(c/c _(L))sin θ]}  (13)

Accordingly, by fixing the grating period (d), the angle of incidence(θ), and the material of the solid member 410 (c_(L)), Eq. (13) yields ameans to determine speed of sound in the fluid 420 based on anidentification of the critical frequency F_(CR).

The significance of the critical frequency F_(CR) is that it is at thisfrequency that the m=1 transmitted wave transforms from a traveling waveand becomes evanescent. This means that it becomes an exponentiallydecaying wave in the fluid 420. In order to conserve energy, the energyof the m=0 traveling wave must be redistributed in some fashion as itbecomes evanescent around the critical frequency F_(CR). It has beenfound that at least a portion of this energy is at least initiallyredistributed to several of the other waves, including the specularlyreflected signal (the m=0 reflected wave), and that this energyredistribution can be detected as an increase in amplitude of therespective signal and used to identify the critical frequency F_(CR). Asdescribed above, identification of the critical frequency F_(CR) leadsto a determination of a value for c in Eq. (13). A similar phenomena hasbeen observed utilizing polarized light incident on a diffractiongrating, as described in U.S. Pat. No. 5,502,560 to Anderson, which ishereby incorporated by reference.

While it has also been found that, in certain circumstances, asignificant portion of the energy of the m=1 transmitted wave isultimately transferred to the m=0 transmitted wave, thereby permittingdetermination of the critical frequency via direct observation of thetransmitted m=0 wave, there is an advantage in observing a wavereflected from the grating 450. Observation of a reflected wave permitsconstruction of a one-sided sensor that does not require directdetection of ultrasound that has been transmitted through the fluid 420.

Accordingly, in one aspect the invention provides a sensor wherein apulser (see FIG. 13) excites a send transducer 430 on the solid side ofa diffraction grating at a solid-fluid interface. A suitable excitationis a tone burst signal at a preselected frequency. A receive transducer440 receives a reflection from the grating 440. The response at thereceive transducer 440 is passed through a receiver and digitizer and toa computer. The frequency is then incremented and the process repeateduntil a desired spectrum of amplitude versus frequency is obtained. Thecomputer corrects the amplitude to account for any variation in receivergain and for variations attributable to the frequency dependence of thetransducers. From the spectrum, the computer applies a peak pickingalgorithm to identify the frequency corresponding to a relevant peak inthe amplitude values. A suitable algorithm is to select an appropriatefrequency window (for example guided by an expected range for thecritical frequency calculated from Eq. 13 based on an expected range forspeed of sound in the fluid 420) and to select the frequencycorresponding to the largest detected amplitude in that window. Theidentified frequency is than compared to a calibration database or Eq.13 is used to determine speed of sound in the fluid 420 based on theknown parameters of the system.

The transducers 430, 440 are preferably wide bandwidth transducer so asto allow a large frequency sweep, for example one or more of transducers430, 440 having a bandwidth, measured as the full width at half maximum,at least about equal to 50% of the center frequency, more preferably atleast about 60-70%. Suitable transducers have piezoelectric transducerelements and are commercially available from a variety of manufacturers,for example Xactex Corp in Pasco, Wash. Alternatively or in addition,multiple transducers of varying center frequencies can be used tocollect the desired frequency spectrum.

It has been found that the detected peaks have a finite width (see e.g.FIGS. 14-16), suggesting that the evanescent transition effectivelyoccurs over a range of frequencies. The width of the peak has been foundto increases as the number of grooves of the grating incident with theultrasound decreases. In other words, narrower or sharper peaks areobserved when a greater number of grooves are illuminated withultrasound. Accordingly, the ability to accurately resolve the frequencyvalue corresponding to the peak, the value used to correlate with speedof sound in the fluid as described above, increases with increasingnumber of grooves illuminated. The number of grooves illuminated isrelated to the relative size of the operational face of transducer 430and the angle of incidence θ, with a larger face providing a beam ofgreater cross sectional area and a larger angle θ providing greatercross sectional area of illumination. In one aspect at least about 20grooves of grating 450 are illuminated. In other aspects at least 30-50are illuminated.

However, it may not be practical to have a face of transducer 430 beingtoo large, because it is advantageous to have the distance D_(S) betweenthe transducer 430 and the grating 450 sufficient to locate the grating450 in the far field of the transducer 450. The distance from thetransducer face to the value of the near field (calculated below) ischaracterized by regions of constructive and destructive interference.When the distance is greater than the near field distance, theultrasound is fairly uniform, generally lacking such interferenceeffects. The face of the diffraction grating should be in the far fieldof the send transducer 430. The near field length (Nf) for an ultrasonictransducer can be approximated by equation (8)Nf=0.25 D ²/λ  (8)where λ is the wavelength of the ultrasound in the medium (equal tolocal speed of sound divided by the frequency) and D is a dimension ofthe operational face of the transducer. For circular transducers, D willbe the diameter of the face whereas for rectangular transducers D can beeither dimension of the rectangle. For purposes of locating thebeginning of the far field and the location of the send transducer 430,the smaller dimension of a rectangle is chosen. Accordingly, in oneaspect, the distance D_(S) is selected to be at least about equal to thebeginning of the far field, for example in the range of 0.9 to 1.25 ofthe Nf according to Eq. 8 with the smaller dimension of the transducerface used for D.

The selection of the distance D_(R) between the receive transducer 440and the grating 450 is independent of the near field and can be chosento be less than D_(S), for example about 0.25 to 0.75 D_(S), so as tominimize the attenuation of the reflected signal as it travels throughthe solid 410.

The selection of the grating spacing and the configuration of thegrating is determined by calculating the critical frequency using Eq.12, using a known velocity of sound for illustration of the principle orusing an approximate speed for the fluids to be encountered. The choiceof grating spacing and transducer frequency are related using Eq. 12.Eq. 12 also shows that, for two slightly different fluids (i.e.,slightly different concentration of the sugar water solutions presentedbelow) the critical frequency values are more widely separated whenusing a smaller grating spacing. That is, the sensitivity of thevelocity measurement is increased by using a smaller grating spacing.

In order to be effective, the material of member 410 needs to be able tosupport formation of the grating and be compatible with and preferablywithstand prolonged contact with the fluid 420. A wide range of materialmay meet one or more of these objectives such as plastics, ceramics, andmetals such as stainless steel or aluminum. Preferably, though notessentially member 410 is a unitary structure providing substantiallycontinuous material along the acoustic paths between the send transducer430 and the grating 450 and between the grating 450 and the receivetransducer 440.

The selection of the angle of incidence θ depends on the material ofmember 410 and the grating spacing, but will typically range betweenabout 15° and 60°, for example between 25° and 50°, such as about 30°.For some choice of the grating material, it may be possible to use abeam that strikes the grating perpendicularly and monitor the reflectionin a pulse-echo mode using only one transducer.

Observation of a peak at the critical frequency involves a balancebetween the strength of the reflected m=0 signal and the strength of them=1 transmitted longitudinal wave that becomes evanescent. For example,if the m=0 reflected wave is very large and the m=1 transmitted wave issmall, it may not be possible to detect a small change when the m=1 wavebecomes evanescent and a portion of its energy is transferred to the m=0reflected wave. Particularly when observing the m=0 reflected signal,improved detection ability can be achieved when parameters are selectedto decrease the relative amount of ultrasound that is reflected from thegrating 450 in the m=0 wave. Increasing the angle of incidence θ is onemechanism for decreasing the relative amount of reflection, while at thesame time, increasing the amount of ultrasound in the transmitted m=1wave (that will become evanescent and must be transferred to othermodes).

An exemplary embodiment of the present invention was constructed andused to evaluate the speed of sound of sugar water solutions. Asdescribed more fully in commonly owned U.S. Provisional Application Ser.No 60/467,878 filed May 5, 2003 and titled Characterization of Fluidsand Slurries Using Ultrasonic Diffraction Grating Spectroscopy, 5 Mhzsquare transducers, 1.27 cm on a side, having a bandwidth of 50% of thecenter frequency at half maximum were positioned at an angle θ of 30°with respect to the normal as both the send and receiver transducers430, 440. A 5.08 cm diameter half cylinder of stainless steel providedthe diffraction grating with a grating spacing of 300 μm with a 120°included angle. A fluid path provided the acoustic communication betweenthe transducers 430, 440 and the half cylinder, which had flat surfacesmachined at the appropriate 30° angles. The face of the send transducerwas a distance of about 11.2 cm from the center of the grating, ensuringthat the ultrasound was in the far field when it reached the gratingsurface, and the receive transducer face was about 5.7 cm from thegrating.

Exemplary data from a scan over frequency is present in FIGS. 14-16where the fluid 420 is water (FIG. 14), 10% sugar water (FIG. 15), and15% sugar water (FIG. 16). Data were obtained for the “blank” that hadthe same dimensions as the stainless steel block providing the grating,but with a smooth face in place of a grating. To account for variationsin the transducer response over frequency, the following calculation wascarried out over the frequency range:

$\begin{matrix}\frac{{Amplitude}\mspace{14mu}{for}\mspace{14mu}{grating}\mspace{14mu}{at}\mspace{14mu} a\mspace{14mu}{given}\mspace{14mu}{frequency}}{{Amplitude}\mspace{14mu}{for}\mspace{14mu}{blank}\mspace{14mu}{at}\mspace{14mu} a\mspace{14mu}{given}\mspace{14mu}{frequency}} & (14)\end{matrix}$That is, at each frequency, the amplitude for the grating was divided bythe amplitude for the blank.

The reflection coefficients can be determined from measurements with thegrating and blank. Because a transducer measures the pressure of anultrasonic wave (not the intensity), the voltage is proportional to thepressure reflection coefficient. For a flat surface the formulation ofreflection and transmission coefficient is well known and is given inAppendix A to the above referenced Provisional filed May 5, 2003. Usingthis formulation, the reflection coefficients for the blank immersed invarious liquids were calculated. Eq. (14) is modified by multiplying bythe (pressure) reflection coefficients for the blank to yield:

$\begin{matrix}{{RCgrating} = \frac{({Vgrating})({RCblank})}{Vblank}} & (15)\end{matrix}$where Vgrating is the voltage response for the grating, Vblank is thecorresponding voltage response for the blank, and RCblank is thecalculated reflection coefficient for the blank. The values forRCgrating are plotted in FIGS. 14-16.

Data below about 4.5 MHz and above about 6.7 MHz in FIGS. 14-16 isconsidered noisy and unreliable. Nonetheless the location of theindicated peak corresponds well with the predicted critical frequencyvalues both qualitatively and quantitatively.

The amplitude of the peak at the critical frequency also is potentiallya mechanism for determining fluid properties. For example, it isbelieved that the amplitude of the peak is related to the properties ofthe fluid and will show dependence on the particle size when the fluidis a slurry. For example, without intending to be bound by any theory,it is expected that as the m=1 transmitted wave becomes evanescent nearthe critical frequency, a portion of the energy will be scattered andabsorbed by the particles in the slurry and will influence the relativeamount of energy transferred to, and thus the amplitude of, the m=0reflected wave. Accordingly, as between two slurries, the change in peakamplitude is expected to indicate a change in particles size.

Another exemplary embodiment of the present invention was constructedand used to evaluate the particle size and concentration of polystyrenespheres in water. As described more fully in U.S. ProvisionalApplication Ser. No. 60/644,758 titled Effects of Particle Size UsingUltrasonic Diffraction Grating Spectroscopy and filed Jan. 17, 2005, theexperimental setup included an aluminum grating with a grating spacingof 482.6 microns. The grating was formed as triangular grooves with anincluded angle of 110° that were machined a length of 3.6 cm on thefront surface of the aluminum piece.

The send transducer operated at 5 MHz and had a width of 2.54 cm and aheight of 1.8 cm. The send transducer was spaced a distance of 10.2 cmfrom the grating so that the grating surface would be in the far fieldof the transducer. The incident angle was 30°. The receive transducerwas positioned a distance of 3.8 cm from the grating center. When incontact with water, the critical frequency was 3.5 MHz. The wavelengthin water is 0.4 mm at 3.5 MHz. The send transducer frequency wasselected to provide a relatively large effective depth of the evanescentwave into the liquid (water). The large width of the send transducer wasselected to assure that more grooves on the grating would be insonified,leading to a sharper peak.

To avoid oxidation of the aluminum when immersed in water, the gratingsurface was machined onto the front of the aluminum piece and thenanodized to form a 0.0018 cm thick layer of aluminum oxide. The aluminumoxide layer reduces the formation of bubbles that occur for purealuminum. The transducers were affixed to the aluminum piece after itwas anodized.

To account for the frequency response of the two transducers, anormalized transducer response curve was established by taking a spectrawhen the aluminum grating is in contact with air in the region aroundthe critical frequency. This curve was then normalized by dividing by aselected frequency, in this case 2.9 MHz. The slurry data presentedbelow was then normalized to this normalized transducer response curveafter being corrected for amplifier gain.

Polystyrene spheres having diameters between 100 microns and 500 micronswere sieved in order to obtain the following smaller size ranges:

-   -   1. 180-250 microns, average of 215 microns    -   2. 250-300 microns, average of 275 microns    -   3. 300-425 microns, average of 363 microns    -   4. 425-500 microns, average of 463 microns        The distribution of the sizes within each range was not known.        Slurry samples were obtained by mixing the polystyrene sphere        with de-ionized water to form weight percentages ranging from 1%        to 12%. Ultrasonic Diffraction Grating Spectroscopy (UDGS) data        were taken at ambient room temperature, which due to the effects        of the magnetic stirrer, was about 25° C.

Exemplary normalized UDGS data for slurries of water and 215 micronpolystyrene spheres at weight percentages of 0%, 1.1%, 2.3%, 3.9%, 5.4%,6.8%, 8.0%, 9.0%, 10.1%, 11.2%, and 12.1% are shown in FIGS. 17 and 18.Exemplary normalized UDGS data for 275 micron spheres at weightpercentages of 0%, 2.3%, 3.9%, 5.5%, 6.8%, 8.0%, 9.0%, 10.0%, and 11.0%are shown in FIGS. 19 and 20. Exemplary UDGS data for slurries of waterand 363 micron spheres at weight percentages of 2.3%, 3.9%, 6.8%, 9.0%,and 11.2% are shown in FIG. 21. Exemplary UDGS data for slurries ofwater and the 463 micron spheres at weight percentages of 2.3%, 3.9%,6.8%, 9.0% and 11.2% are shown in FIG. 22.

A comparison of results for water and slurries at 11 Wt % for the fourdifferent average particle sizes are shown in FIG. 23. One interestingfeature is that the height of the peak for each slurry sample is lessthan the height of the peak for water. Without intending to be bound byany theory of operation, it is believed that when the evanescent waveinteracts with the particles, some energy is lost. This energy loss isshown by the smaller peak height compared to the peak height for water.Another interesting aspect is that, at the constant weight percentage,the peak height decreases with decreasing particle size. That is, theslurry for the 463 micron spheres has a larger peak height than that forthe 215 microns spheres. This suggests that the slurry for 215 micronspheres absorbs more energy during the evanescent wave transition thandoes the slurry for 463 micron spheres. Each set of data has a peak at afrequency between 3.4 MHz and 3.5 MHz, which is the expected value ofthe critical frequency. (In FIG. 23, 0.06 volts was subtracted from thedata for water and the slurry of 215 micron spheres, so that all of thedata had the same value at 2.9 MHz.).

The UDGS data were analyzed by obtaining the maximum value of the peakheight and subtracting the background value. The background value ineach case was taken to be the value of Vgrating/Vairnorm at 2.9 MHz. InFIG. 24, the peak height minus the background is plotted versus theweight percentage of the slurry for each particle size. It isinteresting to note that there is a linear relationship with weightpercentage. The slopes of the straight lines curve fits are given in thelegend of FIG. 24. It is important to note that there is a sizeabledifference between the data for slurries of 215 micron spheres and thatfor 275 micron spheres. As the particle size increases, the sensitivityseems to decrease, as shown by the plot of slope of the straight linecurve fits versus particle size of FIG. 25.

It is postulated that the area under the peak represent the energy thathas not been absorbed by the particles in the slurry. The reasoning isthat previous observations have shown the signal transmitted into theliquid before becoming evanescent results in a peak having definitewidth. As the frequency decreases, this peak moves to a larger angle,and eventually, one edge of the peak reaches 90° and becomes evanescent.As the frequency is further decreased from this point, more of the peakbecomes evanescent until the peak disappears completely. Thus, it seemsreasonable that the area under a peak in a UDGS spectrum represent theenergy that has not been absorbed by the particles in the slurry.

FIG. 26 shows the area under the peak for the slurries of all fourparticle diameters. The area under the peak was obtained byappropriately taking the sum of the data points within the peak andsubtracting the background due to a trapezoidal area below the peak. Alinear relationship between the area under the peak and the weightpercentage of the slurry is demonstrated by the data. This is expectedsince the effect should be proportional to the number of particles inthe slurry. The slopes of the straight lines are shown on the graph.

FIG. 27 shows a plot of the slopes versus the particle diameter. A thirdorder polynomial is fitted through the data points to aid the eye. Herewe see that the absolute magnitude of the slope increases with thesmaller particle diameter. As the particle diameter increases, the slopeapproaches zero. This would seem to be an appropriate behavior since onemight not expect the same sensitivity for all particle diameters.

FIG. 28 shows exemplary plots of the percentage of ultrasound absorbedversus weight percent for the different particle sizes. The slopes oflinear fit lines are given in the plots. The percentage of ultrasoundabsorbed is calculated relative to the absorption of water. As describedabove, the area under the peak is presumed to represent energy that hasnot been absorbed by the particles. Therefore, the percent of energyabsorbed by the particles is given by equation 16:Percent of energy=(Area for water−Area for the slurry)×100%/(Area forwater) Absorbed  (16)FIG. 29 shows a plot of the percentage of ultrasound absorbed versusparticle diameter. It is interesting to note that this relationship isvery nearly linear.

One question of commercial concern is whether, at a specified weightpercentage, one can differentiate the four slurries. In other words, ata given weight percentage, can you extract particle size? It is to beappreciated that UDGS provides several mechanism of accomplishing thiscorrelation. Peak amplitude correlates with particle size and the areaunder the peak (e.g. with the background subtracted) also correlateswith particle size. Suitable mechanisms for making these correlationsinclude calibration curves, look up tables, or other empiricalcorrelations.

Since the wave becomes evanescent over a range of frequencies as thetransmitted beam in the slurry approaches 90°, there may be a preferencefor using the area under the peak for size identification and absorptionrather than the peak amplitude. However, in general, either method maybe employed. For example, FIG. 23 shows useful differences at 11 Wt %for water and the slurries of the four particle diameters, and FIG. 24shows the differing slopes of the peak height above background, as afunction of particle size. Thus, at any desired weight percentage, thepeak height above background has different values for the four slurries.Similarly, FIG. 26 shows the area under the peak plotted versus theweight percentage. The slopes are different, leading to the conclusionthat the area under the peak has different values for the four slurries,at a given weight percentage.

It is also possible to use UDGS to determine particle size when theweight percentage is unknown. The challenge of this process can be seenin the calibration plot of the grating, shown in FIG. 30 a, whichillustrates the interdependence of peak height above background (PKHB)on the ordinate, the weight percentage, and the particle size. (Asimilar plot can be constructed for the area under the peak versusweight percentage.) For a single measurement of PKHB (e.g. the extendedhorizontal dashed line at 0.6), the value of the particle size is notabsolutely determined. However, if the weight percentage is known,(represented by the dashed horizontal line at 8%), then the particlediameter can be pinpointed precisely (in this example 215 microns).Therefore, in commercial applications it will be important to determinethe weight percentage. In general, a variety of methods may be employedfor determining the weight percentage of a suspension, for example anindependent measurement of the density of the slurry. However, the UDGSmeasurements can provide a mechanism for determining the weightpercentage.

For example, weight percentage can be empirically derived from velocitymeasurements. It has been shown that, for homogenous liquid, thevelocity of sound can be obtained by measuring the critical frequencyand also by the peak height of the received signal above background.(PKHB) Thus, by employing a prior calibration, it may be possible todetermine the velocity of sound in the slurry liquid from a measurementof the critical frequency and/or from the absolute value of the PKHB.

Another mechanism is to use two different UDGS measurements. An examplewould be to use two ultrasonic gratings of different design, for exampledifferent material, grating shape, or grating spacing. FIG. 30 b depictsexemplary calibration plots for such a second different grating. Takinga UDGS measurement from both the aluminum grating (FIG. 30 a) and thesecond grating (FIG. 30 b) provides a mechanism to yield both theparticle size and the weight percentage. For example, assume ameasurement with the first grating (FIG. 30 a) yields a PKHB of 0.6, anda measurement on the same sample with the second grating (FIG. 30 b)yields a PKHB of 0.5. Since the sample is the same, the weightpercentage and the particle size must be the same for both gratings.Since only the 215 micron line is intersected by the two dashedhorizontal lines, the particle size must be 215 microns and the weightpercentage must be 8%.

More generally, a linear relationship between the PKAB for a grating 1(denoted as y) and the PKAB for a grating 2 (denoted as z) can bedeveloped by expressing a straight line for a specified particle size asfollows:y=m ₁ x+b ₁ for grating 1  (17)and z=m ₂ x+b ₂ for grating 2  (18)The quantity x represents the weight percentage. Eliminating x from Eq.17 and Eq. 18 results in the following:z=(m ₂ /m ₁)y+b ₂−(m ₂ /m ₁)b ₁  (19)Each particle size has different values of m1, m2, b1, and b2, as shownin FIG. 16. For each particle size, the appropriate values aresubstituted into Eq 19 and the results are plotted in FIG. 31. Thehorizontal and vertical dashed lines in FIG. 31 show the PKAB equal to0.63 for grating 1 and 0.55 for grating 2. These dashed lines intersecton the line for 215 microns. Once the particle size is known, m₁, b₁,and y can be substituted into Eq. 17 to solve for the weight percentagex.

While the foregoing analysis was carried out using the peak height, itis to be understood that the same analysis could be carried out usingthe area under the peak.

In summary, it has been demonstrated that UDGS measurements showdifferences for slurries having different particle size. Both the peakheight above background and the area under the peak can be used to seethe effects of particle size. Thus, it is possible to determine theparticle size, when the weight percentage is known or independentlydetermined.

Furthermore, if the weight percentage of the slurry is not known, thentwo diffraction gratings of different design can be used to obtain boththe particle size and the weight percentage. It is also believed that aUDGS measurements at a different diffraction orders from the samegrating can be used in place of, or in addition to, the measurement fromthe second grating described above to simultaneously obtain weightpercentage and volume fraction.

While an grating spacing of 483 microns and an ultrasonic frequency ofaround 3.5 Mhz was selected for the analysis of the polystyrene slurriesdescribed above, any useful combination of grating spacing andultrasonic frequency can be employed as would occur to those of skill inthe art. For example, when analyzing larger particles, it may bedesirable to use a lower frequency and a larger grating size. Likewise,for smaller particles, a higher frequency and smaller grating size maybe preferable.

Having determined properties of the fluid or slurry using UDGS,additional properties of the sample can be determined as would occur tothose of skill in the art. In one aspect, speed of sound determined inaccordance with the present disclosure is combined with a measuredacoustic impedance value to yield a measure of the density of the fluid.In this aspect, a diffraction grating sensor as described above isprovided in combination with an acoustic impedance sensor.

In one advantageous variation, the acoustic impedance sensor is alsocapable of obtaining data without requiring through transmission throughthe fluid. Such a sensor is described in commonly owned U.S. applicationSer. No. 10/099,412 filed Mar. 15, 2002 and titled Self CalibratingSystem and Technique for Ultrasonic Determination of Fluid Properties,the disclosure of which is hereby incorporated by reference.

Turning now to FIG. 1, a system 20 for analyzing a property of fluid 25and which can be used in conjunction with system 400 described above isdepicted. Fluid 25 can be a gas, liquid, slurry, suspension, paste,emulsion and the like. In preferred forms, fluid 25 is substantially nongasseous and/or includes at least one liquid. In this form, fluid 25might be, for example, a liquid, slurry, or suspension. In furtherpreferred forms fluid 25 has a viscosity greater than about 0.5 cPand/or a density greater than about 0.3 g/cm³.

Ultrasonic transducer 30 is acoustically coupled to a first surface 42of a member 40 comprised of a solid material. In one example, transducer30 is in direct contact with member 40. In other examples, one or morecouplants might be used between transducer 30 and member 40, or they maybe coupled as would otherwise occur to those skilled in the art. Anopposed second surface 44 of member 40 is in contact with the fluid 25.A pulser 22 is electrically coupled to transducer 30 and is operable todeliver input stimulus signal to transducer 30 to cause transducer 30 toemit acoustic energy through solid member 40 and towards fluid 25.Transducer 30 is also operable to produce output signals in response toacoustic energy transmitted from member 40. A processing apparatus 22including receiver 60, digitizer 70, and computer 80, is coupled topulser 22 and to transducer 30. Processing apparatus 22 controlsdelivery of the transducer input signals, receives the output signalsfrom transducer 30, and, as described more fully below, performscalculations to determine properties of fluid 25 as a function of thetransducer output signals.

In operation, pulser 50 generates and delivers a short duration stimulusto transducer 30. Transducer 30 responds to the stimulus by emitting alongitudinal wave pulse of ultrasound into member 40. This ultrasonicpulse reflects between surfaces 44 and 42 producing a series of pulseechoes at transducer 30. This resulting echo series will be ofsuccessively diminishing echo amplitude because each successive echowill have reflected from the solid fluid interface at surface 44 onetime more than the previous echo. An exemplary plot of echo magnitudeversus time after the initial pulse, illustrating echoes 1-5 of adiminishing series of echoes, is shown in FIG. 7.

Transducer 30 responds to the echoes by producing an output signalproportional to the echo amplitude that is amplified by receiver 60,digitized by digitizer 70 and passed to computer 80. Computer 80includes programming instructions encoded on fixed and/or removablememory devices 84, 86, respectively, to select a peak echo amplitude forthe series echoes and to determine the average decay rate of the peakecho amplitudes with increasing echo number in the echo series.Alternatively, computer 80 can be at least partially hard wired withdedicated memory devices and configured to execute logic according tothe present invention. Computer 80 is operatively coupled to display 82to output selected information about fluid 25 integrated with transducer30.

Preferably a number of echo amplitudes, for example 5 or more, spanninga range of echo numbers are used in computing the decay rate. In onepreferred form, computer 80 is programmed to first compute the fastFourier transform (FFT) of the digitized signal, converting it from thetime domain to the frequency domain and then determine the peakamplitude at a selected frequency, where the frequency is selected tobe, for example, the center frequency of transducer 30. In a stillfurther preferred form, the process is repeated for a number of pulsesfrom transducer 30, and the average decay rate of the peak echoamplitudes is determined for each repetition. A rolling average of theresulting set of average decay rates is then determined.

The determined average decay rate can be expressed as the slope of theline of the natural log of echo amplitude versus echo number (mF). Anexemplary plot of log echo amplitude versus echo number with a line fitto the exemplary data is shown in FIG. 8. Utilizing this expression ofthe average decay rate, computer 80 calculates the reflectioncoefficient for the fluid-solid interface (RCfluid) according toequation (1)RCfluid/RCcalib=e ^((mF−mC))  (1)where mC is the slope of the natural log of echo amplitude versus echonumber determined by replacing the fluid 25 with a calibration fluid,and RCcalib is the calculated reflection coefficient for the fluid-solidinterface when the fluid is the calibration fluid. The values forRCcalib and mC are stored in memory 84 and/or 86, and the value forRCcalib is calculated in advance according to equation (2)RCcalib=(Zcalib−Zsolid)/(Zcalib+Zsolid)  (2)where Zcalib is the acoustic impedance of the calibration fluid andZsolid is the acoustic impedance of the solid member 40.

Instead of calculating two slopes as given by equation (1), anequivalent processing technique is to divide the output from transducer30 received for each echo by the corresponding value for the calibrationfluid (i.e. water) to yield a normalized echo amplitude (NA) for eachecho number. The slope of the plot of natural log of these normalizedecho amplitudes versus echo number (mNA) is then used to calculate theratio of the reflections coefficients by equation (1a):RCfluid/RCcalib=e ^(mNA)  (1a)As would be apparent to those of skill in the art, substitution of mNAfor mF≧mC in equation (1) is mathematically and theoreticallyequivalent, but by eliminating the subtraction of separate slopes, hasthe potential to minimize the propagation of rounding and measurementerrors.

From the fluid specific reflection coefficient (RCfluid), computer 80calculates the acoustic impedance of the fluid (Zfluid) according toequation (3)Zfluid=Zsolid (1−RCfluid)/(1+RCfluid)  (3)where Zsolid is the acoustic impedance of the solid member 40.

From the acoustic impedance of the fluid (Zfluid), computer 80calculates a physical property of the fluid. The density of the fluid(ρ_(F)) is calculated according to equation (4)ρ_(F) =Zfluid/Vfluid  (4)where Vfluid is the speed of the sound in the fluid. An indication ofthe fluid density is then produced on display 82.

In one form, the speed of sound (Vfluid) is determined by performance ofa time-of-flight measurement on the fluid. A time-of-flight measurementis accomplished by measuring the time it takes an ultrasound pulse totravel a known distance through the fluid 25. The speed of sound(Vfluid) is then determined by dividing the known distance by thedetermined transit time. FIGS. 2 and 3 schematically illustrate devices102 and 104 for performing time-of-flight measurements that can form aportion of system 20. In the FIG. 2 embodiment, a pair of transducers110, 112 are arranged in pitch-catch mode and measure the time it takessound to travel from transducer 110 to transducer 112. In the FIG. 3embodiment, a single transducer 114 is arranged relative to a surface116 in pulse-echo mode for measuring the time it takes sound to travelfrom transducer 114 to surface 116 and back. Because the ultrasoundtravels through the fluid in a time-of-flight measurement, it ispreferred to use a lower frequency of ultrasound in the time-of-flightmeasurement than in the echo measurement to minimize attenuation ofultrasound in the fluid during the time-of-flight measurement. Inparticular forms, the time-of-flight measurement is performed at afrequency below about 1 MHz.

In another form, the speed of sound is determined via ultrasonicdiffraction grating spectroscopy as described above. As will beappreciated by those of skill in the art, the use of ultrasonicdiffraction grating spectroscopy eliminates the need to perform a timeof flight measurement and provides the ability to measure a value forspeed of sound that does not require traversal of a section of thefluid.

One variation of system 20 is depicted in FIG. 4. System 24 includesboth a shear wave transducer 34 and a longitudinal wave transducer 36.Transducers 34 and 36 are each coupled to pulser 50 and processingapparatus 22 via a multiplexer 38. In this variation, processingapparatus 22 is programmed to simultaneously or sequentially cause shearwaves and longitudinal waves to be reflected through member 40.Processing apparatus 22 is programmed to receive the output oflongitudinal transducers 34 when longitudinal waves are being reflectedthrough member 40 and to determine fluid density information asdescribed above with respect to system 20. Alternatively, longitudinalwave transducer 34 can be omitted with fluid density determined by anyother means known in the art.

Processing apparatus 22 is also programmed to determine one or moreadditional properties of the fluid utilizing the response of transducer36 to the reflected shear waves in combination with the determineddensity information. The response from shear transducer 36 is process asdescribed above with respect to transducer 30 to calculate the acousticimpedance of the fluid according to equations (1)-(3), where the valuesused in equation (1)-(3) and the determined acoustic impedance (Zfluid)appropriately correspond to values for shear waves.

In one preferred form, the additional properties determined from theshear wave acoustic impedance depend on the properties of the fluidbeing interrogated. The propagation of a shear waves in liquids isdescribed in J. Blitz, Fundamentals of Ultrasonics, 2^(nd) Edition,Plenum Press, New York, 1967, pp. 130-134, which is hereby incorporatedby reference in its entirety. As described in Blitz, both the viscosity(η) and the shear modulus (G) are parameters in differential equationsinvolving the rate of change of the shear strain, the pressure, and thepressure time dependence for shear wave propagation. The relaxation time(τ) for liquids is defined as the viscosity (η) divided by the shearmodulus (G). Where the relaxation time is small such that the termsinvolving G can be ignored, the viscosity of the fluid (η) is calculatedin accordance with equation (5).Zfluid=(ωρ_(F)η/2)^(0.5)  (5)where ω is the radial frequency of the shear wave and ρ_(F) is thedetermined fluid density. Exemplary small relaxation times for this forminclude relaxation times less than about 10⁻⁹ and more preferably on theorder of about 10⁻¹². An equivalent formulation for determining fluidviscosity by combining equations (3) and (5) and substituting for Zsolidis given in equation (5a).

$\begin{matrix}{( {\rho_{F}\eta} )^{0.5} = {\rho\; s\mspace{14mu}{c_{TS}( \frac{2}{\omega} )}^{0.5}( \frac{1 - {RCfluid}}{1 + {RCfluid}} )}} & ( {5a} )\end{matrix}$where ρs is the density of the solid and C_(TS) is the shear wavevelocity in the solid.

For fluids 25 where the value of ωτ>>1, shear modulus (G) or the shearvelocity in the fluid (c_(tf)) can be calculated according to equations(6) and (7).Zfluid=(ρ_(F) G)^(0.5)  (6)Zfluid=(ρ_(F) C _(tf))  (7)Exemplary values for ωτ according to this form include values greaterthan about 3 and more preferably greater than about 11.

In other forms or where these simplifications are not utilized,additional fluid properties can be determined by solving Blitz'sdifferential equations numerically and/or by any means known in the art.

One application for the invention, depicted in FIGS. 9 and 9A, includesa fluid sensor system 300 that can be inserted in a pipeline and coupledto appropriate electronics to determine fluid properties in accordancewith the present invention. System 300 includes a spool piece 310 thatis adapted to be inserted in a section of a process pipeline whichconveys a fluid in need of characterization. The spool piece 310includes a shear wave transducer 320 and a longitudinal wave transducer330. The transducers 320, 330 are configured to detect multiplereflections of an echo series as described above. The spool piece alsoincludes a solid member 340 having a diffraction grating 346, a sendtransducer 342 and a receive transducer 344. The grating 346 ispositioned to contact the fluid contents of the spool piece 310, andtransducers 342, 344 are positioned to perform diffraction gratingspectroscopy as described above. Each of the transducers 320, 330, 342,344 are coupled to one or more computer(s) (not shown) or similarprocessing device(s) for performing the calculations described above fordetermining desired properties of the fluid such as speed of sound,acoustic impedance, density, and viscosity. Additional sensors can alsobe included in the spool piece 310 such as a thermocouple, pressuretransducers, and/or a flow meter for measuring fluid temperature,pressure drop, and/or flow velocity.

Transducers useful for forming and receiving the ultrasound pulse echoseries in practicing the present invention can operate in the range ofabout 0.5 to 20 MHz, more preferable between about 1 and 10 MHz, andmost preferably about 5 MHz. In certain applications of the invention,the thickness T of member 40 will be predetermined, and depending on thewavelength of ultrasound in the member 40, the ratio of thickness T towavelength could be significant, for example greater than about 0.05. Asone example, it is contemplated that member 40 would be the existingwall of a stainless steel pipe or container about 0.15 inches thick. Forat least some selected ultrasonic frequencies, the wavelength ofultrasound will be significant relative to the wall thickness.

Where the length of the pulse in the member 40 is a concern, a broadbandultrasound pulse can be used. Pulser 50 inputs a square wave or spikeinput to transducer 30, where the non-sinusoidal input has a durationless than the time it takes the transducer to perform a half cycle atthe transdcuer center frequency (give by the inverse of the frequency ofthe transducer). The transducer 30 responds to this short input stimulusby emitting an ultrasonic pulse into member 40 of short duration, forexample on the order of about 3-4 wavelengths in length. In this manner,the length of the ultrasound pulse in member 40 can be minimized and theechoes detected by transducer 30 can be readily resolved, because thepotential for overlap is typically reduced.

In another form, because of the materials desired for solid member 40and fluid 25, the acoustic impedance ratio Zsolid/Zfluid will besignificant, for example, greater than about 5 or 10. In this form, theultrasound pulse is preferably detected as it undergoes a large numberof reflections between surfaces 42 and 44 of member 40, for example morethan about 10 reflections, preferably about 15-20 reflections. Themultiple reflections serve to amplify the effect of small changes inproperties of fluid 25. This amplification occurs because the amplitudeof the pulse is diminished in accordance with the reflection coefficient(RCfluid) with each successive reflection with surface 44. Also, becausethe higher echoes undergo more reflections with surface 44 and becausethe reflection coefficient (RCfluid) is a function of fluid properties,the effect of changes in these fluid properties are more pronounced inthe higher echo numbers. Consequently, in one form of the invention, itis preferred that at least some of the higher number echoes are used incomputing the decay rate.

In further forms, where reduction of the adverse effects of divergenceand/or attenuation is of concern, selection of transducer 30 and member40 dimensions and properties can be of particular interest. For example,the near field can be considered the region immediately in front of anultrasonic transducer where the sound beam is does not diverge andsignal loss is at a minimum. The near field length (Nf) for anultrasonic transducer can be approximated by equation (8)Nf=0.25D ²/λ  (8)where λ is the wavelength of the ultrasound in the medium (equal tolocal speed of sound divided by the frequency) and D is a dimension ofthe transducer face 32 associated with the member 40. For circulartransducers, D will be the diameter of the face 32 whereas forrectangular transducers D is selected to be the larger length dimensionof the rectangle for purposes of locating an approximate end to the nearfield. In one form of the invention, the near field of the transducer 30is selected to encompass one or more of the reflections used tocalculate the decay rate. In a preferred form, a plurality of the echoesused to calculate the decay rate are within the near field lengthestimated by equation (8). In a further preferred form, the majority ofthe echoes used to calculate the decay rate are within this length. Mostpreferably, substantially all of the echoes are within this length.

From an examination of equation (8) one possibility for increasing thenear field length is to increase the frequency of the ultrasound.However, there is a practical limit to the effectiveness of thisapproach, at least because losses due to attenuation of the ultrasoundgenerally increase with increasing frequency. The near field length istherefore preferably maintained at a desired relative length byadjusting the ratio of the size of transducer size D to thickness T.Increasing the transducer size D increases the near field length whereasdecreasing T decreases the pathlength of the echoes, allowing moreechoes to be detected inside a given near field length. It is to beunderstood that the pathlength for each echo is the distance the pulsetravels for each reflection (2T) times the echo number (the first echohas a pathlength of 2T, the second 4T, the third 6T, etc.). While anyratio can be utilized as would occur to those of skill in the art, inone form of the invention the ratio of D/T is preferably greater thanabout one. In other forms, the ratio D/T is about 2 or above.

An advantage is realized by using the decay rate of the echo amplitudes(represented by the two slopes mF and mC) in determining fluidproperties. It has been found that, unlike the absolute magnitude ofindividual echo amplitudes, the slope of echo amplitude versus echonumber is substantially independent of characteristics of the ultrasoundpulse used to create the echoes. This independence was confirmedexperimentally utilizing a 1 inch diameter longitudinal transducer incontact with a 0.25 inch thick stainless steel plate. The transduceroperated at 5 MHz and the opposed surface of the plate was in contactwith water.

In one set of experiments, the width of a −300 volt square wave input tothe transducer was varied. It was found that, while the absolute valueof the 6^(th) echo amplitude changed by about 21% when the width of thevoltage input was changed from 102 nanoseconds to 68 nanoseconds, theslope of the natural log of the FFT amplitude versus echo number changedby less than 0.1%.

In a second set of experiments the voltage of a 100 nanosecond squarewave input was changed from −300 volts to −50 volts and the slopes ofthe amplitude versus echo number log plots were determined. While themagnitude of the voltage input was decreased by a factor of six, thecalculated slope of the log of amplitude versus echo number changed byless than 2%.

As described above with respect to FIGS. 9 and 9A, in one applicationthe transducer 30 and solid member 40 are provided as a spool piece thatis fixed in place in a pipeline. In other applications, preexisting pipeor container walls as utilized as member 40, and transducer 30 isconfigured as a clamp-on sensor that can be retrofit to existingequipment and/or readily moved from one pipeline or container to thenext. In these latter applications, where preexisting walls providemember 40, the use of the slope of the log of echo amplitude versus echonumber is particularly advantageous.

Turning now to FIG. 5, an exemplary clamp on sensor 220 for use on apipeline is illustrated. Sensor 220 includes an ultrasonic transducer130 which is used in place of transducer 30 in system 20. Transducer 130is curved to correspond to the outer diameter of pipe 140, andtransducer 130 is held to the outside surface of a pipe 140 with clamps150 that extend around pipe. Transducer 130 is generally rectangularwith its longer dimension D oriented parallel to the flow direction ofthe pipe 140. This longer length D is preferably greater than the pipewall thickness T for the reasons described above. As one example, acurved rectangular transducer 0.4 inches by 1 inch could be chosen for astainless steel pipe with an outside diameter of 2.375 inches and a wallthickness of 0.15 inches. An acoustic couplant, not shown, is optionallyprovided between transducer 130 and pipe 140. It is to be understoodthat the strength of any particular signal from transducer 130 mightdepend on, for example, the pressure exerted by clamps 150, which inturn could depend on additional factors, such as the care with whichtransducer 130 is attached to pipe 140. However, the slope of the log ofecho amplitude versus echo number would be relatively independent ofvariables such as connection pressure, leading to increased accuracy ofthe device.

In use, clamp on sensor 220 can be calibrated with any fluid present inpipe 140. If the pipe is empty, air can be the calibration fluid. If thepipeline is conveying a process fluid, the process fluid can be thecalibration fluid. Subsequent changes in the process fluid can then bequantitatively or qualitatively determined according to the presentinvention.

It is to be understood that, while in a retrofit system such as system220, the existing material of the pipe or container wall dictates thechoice of solid material used, a wide variety of materials can serve asthe member 40 as would occur to those of skill in the art. Exemplarymaterials for solid member 40 include aluminum, stainless steel, fusedquartz, and plastics. Preferably member 40 is non-porous is does notabsorb fluid 25. In particular applications, such as food processing andthe transport of toxic material, stainless steel or other non-corrosivematerials are preferred materials for solid member 40.

In a further variation, data transmission between computer 80 andtransducer 30 can be achieved wirelessly by provision of appropriatewireless communication devices.

It is also to be understood that another embodiment is a uniquetechnique to determine fluid properties wherein an ultrasonic transducer30 is provided on a surface 42 of a solid member 40 having an opposedsecond surface 44 in contact with the fluid 25. This technique caninclude delivering an ultrasonic pulse through the solid member,detecting a multiplicity of pulse echoes caused by reflections of theultrasonic pulse between the solid-fluid interface and thetransducer-solid interface, and determining the decay rate of thedetected echo amplitude as a function of echo number. The determineddecay rate is compared to a calibrated decay rate to determine anacoustic property of the fluid. In one form, the speed of ultrasound inthe solid is also determined and the fluid viscosity and/or the fluiddensity is determined as a function of the speed of ultrasound and thedetermined acoustic property.

Another form of the invention is a system for determining a property ofa fluid comprising a diffraction grating in contact with the fluid andformed on a first member; an interrogation device providing ultrasoundthat passes through at least a portion of the first member and isincident on the diffraction grating at an angle of incidence; a detectorfor capturing a reflection spectrum from the diffraction grating whenthe ultrasound is incident on the diffraction grating, the reflectionspectrum including a diffraction order equal to zero; and a processingdevice receiving an output of the detector for determining a valuecorresponding to a property of the fluid based upon the reflectionspectrum. The processing device can be operable to determine the valueby selecting a wavelength corresponding to a peak in the reflectionspectrum. The property of the fluid can be speed of sound in the fluid.The interrogation device can include a transducer face in acousticcontact with the first member. The transducer face can be spaced fromthe diffraction grating a distance at least about equal to D²/(4λ) whereD is the smallest dimension of the transducer face and λ is thewavelength of the ultrasound in the solid material. The grating caninclude at least 20 grooves with a period between about 50 μm and about500 μm. The grating can include grooves with a triangular cross section.The angle of incidence can be between about 25 and about 50°. The firstmember can be stainless steel. There can also be a second membercomprised of solid material and having first and second opposed surfaceswith a transducer in acoustic contact with the first surface and thesecond surface in contact with the fluid. The transducer in acousticcontact with the first surface of the second member can be coupled to aprocessing device operable to determine a value corresponding toacoustic impedance of the liquid from a decay rate of ultrasoundreflected between the first and second surfaces of the second member.

Still another form is a method for determining a property of a fluidcomprising interrogating a diffraction grating in contact with the fluidwith ultrasound at an angle of incidence by passing the ultrasoundthrough a member comprised of solid material and having the diffractiongrating having a grating period formed on a face thereof, receiving aresponse to the interrogating wherein the response includes a reflectionspectrum of ultrasound reflected at a predetermined angle relative tothe normal of the diffraction grating; and determining first valuecorresponding to a property of the fluid by selecting a peak in thereflection spectrum. Determining the value can include comparing thevalue to a value for a calibration sample. The interrogation can beperformed with a transducer having a face spaced from the grating adistance at least about equal to D²/(4λ) where D is the smallestdimension of the transducer face and λ is the wavelength of theultrasound in the solid material. A second value corresponding todensity of the fluid can be determined from the first value and a thirdvalue corresponding to acoustic impedance. This third value can bedetermined by reflecting an ultrasound pulse a multiplicity of timesbetween a pair of opposed surfaces one of which is in contact with thefluid.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be consideredillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges, equivalents, and modifications that come within the scope ofthe inventions described herein or defined by the following claims aredesired to be protected. Any experiments, experimental examples, orexperimental results provided herein are intended to be illustrative ofthe present invention and should not be construed to limit or restrictthe invention scope. Further, any theory, mechanism of operation, proof,or finding stated herein is meant to further enhance understanding ofthe present invention and is not intended to limit the present inventionin any way to such theory, mechanism of operation, proof, or finding. Inreading the claims, words such as “a”, “an”, “at least one”, and “atleast a portion” are not intended to limit the claims to only one itemunless specifically stated to the contrary. Further, when the language“at least a portion” and/or “a portion” is used, the claims may includea portion and/or the entire item unless specifically stated to thecontrary.

1. A system for determining a property of a multiphase fluid comprising:at least one diffraction grating in contact with the fluid; at least oneinterrogation device providing ultrasound incident on the at least onediffraction grating; at least one detector for capturing at least onereflection spectrum from the at least one diffraction grating when theultrasound is incident, on the at least one diffraction grating; and aprocessing device receiving an output of the at least one detector fordetermining at least one value corresponding to a property of themultiphase fluid based upon the at least one reflection spectrum.
 2. Thesystem of claim 1 wherein the multiphase fluid includes solids in aliquid, and the at least one determined value corresponds to a size or aconcentration of solids in the liquid.
 3. The system of claim 2 whereinthe processing device is operable to calculate a size or a concentrationof the solids in the fluid based on a value corresponding to anintegration or a peak height in the reflection spectrum.
 4. The systemof claim 1 wherein the at least one diffraction grating includes firstand second different diffraction gratings.
 5. The system of claim 4wherein the diffraction gratings have different grating periods.
 6. Thesystem of claim 4 wherein the diffraction gratings are comprised ofdifferent solid material.
 7. The system of claim 4 wherein themultiphase fluid includes solids in a liquid and wherein the at leastone value includes a first value and a second value, the first valuecorresponding to a size of the solids in the liquid and the second valuecorresponding to a concentration of the solids in the liquid.
 8. Thesystem of claim 1 wherein the at least one reflection spectrum includesat least two different diffraction orders.
 9. The system of claim 8wherein the multiphase fluid includes solids in a liquid and wherein theat least one value includes a first value and a second value, the firstvalue corresponding to a size of the solids in the liquid and the secondvalue corresponding to a concentration of the solids in the liquid. 10.A system for determining a property of a fluid comprising: first andsecond different diffraction gratings in contact with the fluid; atleast one interrogation device providing ultrasound incident on thefirst and second diffraction gratings; at least one detector forcapturing first and second reflection spectra from the first and seconddiffraction gratings, respectively; and a processing device receiving anoutput of the at least one detector for determining fluid propertiesbased on the first and second reflection spectra.
 11. The system ofclaim 10 wherein the fluid is a multiphase fluid.
 12. The system ofclaim 11 wherein the fluid includes solids in a liquid and theprocessing device determines values corresponding to size andconcentration of the solids in the liquid.
 13. A method for determininga property of a multiphase fluid comprising: interrogating at least onediffraction grating in contact with the multiphase fluid with ultrasoundat an angle of incidence by passing the ultrasound through a membercomprised of solid material and having the diffraction grating having agrating period formed on a face thereof, receiving a response to theinterrogating wherein the response includes a reflection spectrum ofultrasound reflected at a predetermined angle relative to the normal ofthe diffraction grating; and determining a first value corresponding toa property of the multiphase fluid from the reflection spectrum.
 14. Themethod of claim 13 wherein interrogating at least one diffractiongrating includes interrogating first and second different diffractiongratings.
 15. The method of claim 14 wherein the first and seconddiffraction gratings have a different diffraction period.
 16. The methodof claim 13 wherein the received response includes different diffractionorders.
 17. The method of claim 16 wherein the received responseincludes diffraction orders between zero and three inclusive.
 18. Amethod for determining a property of a multiphase fluid comprising:providing a diffraction grating having a grating period formed on amember comprising solid material; providing ultrasound through themember and incident on the diffraction grating at an angle of incidence;capturing reflections from the diffraction grating while the diffractiongrating is in contact with the fluid and the ultrasound is incident onthe diffraction grating; and determining a first property of themultiphase fluid from a first peak in the spectrum of the capturedultrasound.
 19. The method of claim 18 further comprising capturingreflections from a second different diffraction grating.
 20. The methodof claim 18 further comprising determining a second property of themultiphase fluid from a second peak in the spectrum of the capturedultrasound.